According to Wikipedia there are just 150 ppm of sulphur dioxide and 20 ppm of water in Venus' atmosphere. At the same time it is known that there is a considerable amount of sulphuric acid in the clouds. Venus' clouds are made of concentrated sulphuric acid, if I understand it correctly. This would suggest that the amount of sulphuric acid on Venus should be higher than the amount of water (otherwise sulphuric acid would absorb the water since it is strongly hygroscopic).

Also, if data cited on Wikipedia are from some spectrographic measurements, this may be affected by some bias: above the clouds there is probably a much lower concentration of sulphuric acid vapors than under the clouds.

I ask especially concerning possible colonization and local resource utilization on Venus. Two of the limiting substances are water and hydrogen, and a lot of $H_2SO_4$ would be helpful as a source of both hydrogen and water.

To be more specific with the question. 20 ppm of water means $9.6 \times 10^{15} \text{ kg } H_2O$ in total (Venus' atmosphere's mass is $4.8 \times 10^{20} \text{ kg}$). Is there less or more sulphuric acid?

Just for scale ... the amount of various sources of carbon on Earth follows:

 CO2 in Earth atmosphere     0.810e+15 kg
 Earth coal reserves         0.9e+15 kg (proven recoverable)
 Earth biosphere             1.9e+15 kg 
 CO2 in Earth sea water     36e+15kg

First of all, thanks to @Robert Walker, who informed us about this dissertation of Yeon Joo Lee in his comment on the other answer.

In this article on page 14, we can see figure 1.3 with the vertical structure of Venus' cloud layers with mass loading profiles.

From this figure we can detect a cloud layer at about 48 to 50 km height with roughly a mass loading of 100 mg/m$^3$.

At 50 to 54 km height there is a mass loading of about 10 mg/m$^3$ and from 54-66 km i guess a mean mass loading of roughly 5 mg/m$^3$ would be suitable.

All these mass loadings from the 3 mentioned layers added up equals a roughly 3 km thick layer with a mass loading of about 100 mg/m$^3$.

To calculate the volume of this assumed layer we use Venus' surface area which is 4.6.10$^1$$^4$ m$^2$ and enlarge this area to 5.10$^1$$^4$ m$^2$ for the layer.

Accordingly the total weight of the droplets in this layer is 5.10 $^1$$^4$ m$^2$ x 3.10$^3$ m x 10$^-$$^4$ kg/m$^3$ = 15.10 $^1$$^3$ kg.

Assuming 80% of H$_2$SO$_4$ in these droplets gives a total of 12.10 $^1$$^3$ kg H$_2$SO$_4$.
According to the question the total amount of water is: 9.6.10 $^1$$^5$ kg H$_2$O

So there is much less sulfuric acid than water !

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OK, so I found something which more or less answers the question. Presentation Aerosols and Clouds on Earth and Venus from Owen Brian Toon of Department of Atmospheric and Oceanic Sciences and the abstract of BEYOND SULPHURIC ACID – WHAT ELSE IS IN THE CLOUDS OF VENUS?

In short the density of the clouds is about 1-50 $mg / m^3$ of liquid phase. Clouds are formed from 75-90% $H_2SO_4$. $6 \times 10^{13} \text{ kg }H_2SO_4$ is produced each year by photochemical processes.

But still it does not answer exactly what is the total amount.

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    $\begingroup$ Doesn't answer your original question, sorry, but just to say that in the clouds it's much denser. Assuming the droplets that form the cloud layer at 60 km are made of sulfiuric acid then it's an estimated 0.1 grams per cubic meter for the densest layer at 48.5 km, which compares favourably with fog levels on Earth of .05 g/m3 and is at the lower end of the density of cumulonimbus (thunderstorm clouds) at 0.1 - 0.3 g/m3. See page 14 of this paper: mps.mpg.de/phd/theses/… $\endgroup$ – Robert Walker Sep 25 '16 at 12:28
  • $\begingroup$ @RobertWalker Thanks to you i could answer the question ! $\endgroup$ – Cornelisinspace May 26 '18 at 9:58

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